Capacitive sensing is a technology for detecting proximity, position, etc., based on capacitive coupling effects. Capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. Capacitive coupling is typically achieved by placing a capacitor in series with the signal to be coupled. Capacitive sensing is becoming increasingly popular as a human-device interface technology, for example laptop trackpads and touchscreen displays used in many consumer electronic products, as well as proximity sensing, for example a remote keyless entry system for an automobile, but it is certainly not limited to these applications.
For example, a capacitive touchscreen panel is a sensor typically made of glass coated with a material such as indium tin oxide. This type of sensor is basically a capacitor in which the plates are the overlapping areas between the horizontal and vertical axes in a grid pattern. Since the human body also conducts electricity, a touch on the surface of the sensor will affect the electric field and create a measurable change in the capacitance of the device. These sensors work on proximity, and do not have to be directly touched to be triggered.
In general, there are a number of conventional methods for capacitive sensing. These methods are based on either charge redistribution principle, direct measurement of current through the sensed capacitance, measurement of RC time constant, or measurement of frequency in an oscillator containing the sensed capacitance. Sigma-delta capacitive sensors are a sub-category of the charge redistribution sensors. In many applications, it is important for the capacitive sensor to be immune to interference and noise at frequencies other than the sensor operating frequency. In other words, the capacitive sensor input should exhibit a high level of selectivity and reject all frequencies other than the operating frequency. It is also important that the sensors electromagnetic emission is low and it is band-limited around the operating frequency. The existing capacitive sensors are lacking the above characteristics.